Throughout this discussion of the physis, the images created by the late Professor Robert Schenk of the University of Berne, generously donated to the public domain, via the AO Foundation, have proved an invaluable resource.

Postnatally, the next skeletal event is the invasion of the cartilaginous end caps (epiphyses) by blood vessels, resulting in the formation of the epiphyseal ossific nuclei (secondary ossification centers).

Trueta, of Oxford, England, first described the vascularity of the physis. He noted that no significant vessel pierced the physeal disc and that there exist no significant anastomoses between the epihyseal and metaphyseal vascular trees within the bone.

The effect of this is that any shearing disruption of the physis, which usually occurs through the zone of hypertrophy, leaves each of the separated portions of the physis with a functional vascular supply.

There are some epiphyses that are totally covered by articular cartilage, notably at the proximal femur and the proximal radius, where the vessels feeding the epiphysis are bound down tightly to the perichondrium of the periphery of the physis.

In such sites, a shear injury of the physis is highly likely to devascularize the epiphysis and thereby deprive the reserve and proliferating zones of the physis of nutrition. This commonly results in physeal growth arrest and avascular necrosis of the affected epiphysis.

The reserve zone of the physis lies adjacent to the bone of the secondary ossific center: it comprises small, scattered round cells, densely nucleated, and with an abundant endoplasmic reticulum, a clear indication that they are actively synthesizing protein. Their function remains obscure – they do not proliferate at any rate that could contribute to the cell populations of the other zones of the physis.

The proliferating zone cells are the fundamental “power-house” of the physis. If they cease to reproduce, for example if deprived of nutrients as a result of lost blood supply to the epiphysis, then longitudinal growth will cease.

As the cells mature, they enlarge: this enlargement is greater in length than in width and it is this increase in their longitudinal dimensions that is the principle factor resulting in longitudinal bone growth.

There is a specialized fibrous area surrounding the periphery of the growth plate, comprising the groove of Ranvier and the perichondrial ring of LaCroix. There are cells in this area that are specialized chondrocytes, which increase laterally by appositional growth, thereby resulting in an increase in the diameter of the physis as maturity progresses.